reserve i, j, m, n for Nat,
  z, B0 for set,
  f, x0 for real-valued FinSequence;

theorem Th1:
  for R being Relation, Y being set st rng R c= Y holds R"Y = dom R
proof
  let R be Relation, Y be set;
  assume rng R c= Y;
  then rng R /\ Y = rng R by XBOOLE_1:28;
  hence R"Y = R"rng R by RELAT_1:133
    .= dom R by RELAT_1:134;
end;
